In recent years, new semiconductor materials (including so-called semi-insulating materials) for implementing semiconductor devices which have special functions and are particularly excellent in specific characteristics such as an RF characteristic, a light-emitting characteristic, and a breakdown voltage characteristic have been under vigorous development. Of the semiconductor materials, e.g., a semiconductor containing an indium phosphide (InP) as a main component thereof is expected to be applied to a next-generation RF device, high-temperature operating device, and the like because the electron mobility and the saturation velocity of electrons are higher than those of silicon (Si) which is a typical semiconductor material.
In general, a power device is a generic name for a device which converts or controls high power and is termed a power diode, a power transistor, or the like. Exemplary applications of the power device include a terminal in a communication system and a transistor disposed at a base station. Examples of the transistor include a HEMT (High Electron Mobility Transistor) and a bipolar transistor. The applications of the power device is expected to be widened in the future.
A typical modular structure used for such applications is obtained by connecting a plurality of semiconductor chips each having a power device embedded therein with wires in accordance with a use or an object and placing the connected semiconductor chips in a single package. For example, a desired circuit is constructed with semiconductor chips and wires by forming the wires on a substrate such that a circuit suitable for the use is constructed and mounting the individual semiconductor chips on the substrate. A description will be given herein below to a transmitting/receiving circuit at a radio base station which uses a Schottky diode and a MESFET.
FIG. 27 is a block circuit diagram showing an internal structure of a conventional base station (base station in a mobile communication system) disclosed in a document (Daisuke Ueda et al., “Radio-Frequency and Optical Semiconductor Devices Exploring New Age of Data Communication, IEICE, Dec. 1, 1999, p.124). As shown in the drawing, the circuit comprises an antenna main body, a switch, a received-signal amplifier, an amplified-signal transmitter, a radio transmitter/receiver, a baseband signal processor, an interface unit, an exchange controller, a controller, and a power supply portion. The received-signal amplifier is composed of two filters and two low-noise amplifiers (LNA) disposed in series. A mixer for mixing an output from a local amplifier with an output from an RF emitter to generate an RF signal is disposed at the radio transmitter/receiver. A power dividing/synthesizing circuit having a driver amplifier, a filter, a middle amplifier, and a main amplifier disposed therein is disposed at the amplified-signal transmitter. There are further provided a baseband signal processor for processing an audio signal, an interface unit, and an exchange controller connected to a network.
At the conventional base station, the main amplifier is so configured as to perform impedance matching by disposing an input matching circuit and a field-effect transistor (MESFET or HEMT) formed by using a GaAs substrate, while disposing a capacitor, an inductor, and a resistor element on each of the input side and output side.
A MOSFET formed on a silicon substrate, a diode, a capacitor, a resistor element, and the like are disposed in the controller, the baseband signal processor, the interface unit, and the exchange controller. Such parts as a capacitor and an inductor which occupy a particularly large area are formed as independent chips.
FIG. 28 schematically shows a structure of a conventional HEMT using an indium phosphide (InP) substrate. As shown in the drawing, an undoped InAlAs layer 502 with a thickness of about 200 nm, an undoped InGaAs layer 503 with a thickness of about 15 nm, an n-InAlAs layer 504 doped with silicon (Si) to serve as a carrier supplying layer with a thickness of about 10 nm, an InP layer 505 serving as an etching stopping layer with a thickness of about 5 nm, an n-InAlAs layer 506 doped with silicon (Si) and having a thickness of about 3 nm, an n+-InAlAs layer 507 doped with silicon (Si) as an n-type impurity at a high concentration and having a thickness of about 200 nm, and an n+-InGaAs layer 508 doped with silicon (Si) as an n-type impurity at a high concentration and having a thickness of about 15 nm are stacked successively on a semi-insulating InP substrate 501 doped with iron (Fe) at a high concentration and having a thickness of about 100 μm. There are further provided ohmic source/drain electrodes 509a and 509b each composed of a TiAu film and provided in mutually spaced relation on the n+-InGaAs layer 508, a Schottky gate electrode 510 composed of WSi and penetrating respective parts of the n-InAlAs layer 506, the n+-InAlAs layer 507, and the n+-InGaAs layer 508 to be in contact with the InP layer 505, and an insulating layer 511 composed of a SiO2/SiNx film for providing a dielectric isolation between the Schottky electrode 510 and the ohmic source/drain electrodes 509a and 509b. 
In the transistor, if a voltage is applied between the source and drain electrodes 509a and 509b, a current flows between the source and the drain. If a voltage is applied between the Schottky gate electrode 510 and the ohmic source electrode such that the Schottky gate electrode 510 has a higher voltage (reverse voltage), the source/drain current is modulated in accordance with the voltage applied to the Schottky gate electrode 510 so that a switching operation is performed.